Method for gettering noble metals from mineral acid solution

ABSTRACT

Silicon is employed as a reducing agent in an acid bath to adsorb noble metals present as contaminants in the acid. In the manufacture of silicon devices for electronic memory and other devices, polonium-210 is adsorbed by silicon getters to reduce soft error rate attributable to alpha particle emissions from the radioactive polonium. The noble metals in addition to polonium which can be plated onto silicon using the disclosed method are gold, silver, platinum, copper, palladium, mercury, selenium and bismuth.

This application is a continuation of application Ser. No. 07/975,789,filed Nov. 13, 1992, U.S. Pat. No. 5,501,767.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of removing noble metal contaminantsfrom a mineral acid bath, and more particularly, in the manufacturing ofpackaged semiconductor devices, a method for removing contaminant Po-210from a heated phosphoric acid bath using silicon as a getter.

2. Description of the Prior Art

In the process of manufacturing large scale integrated circuits (LSI)from a silicon substrate or wafer, phosphoric acid is often used as asolution to selectively remove silicon nitride.

Phosphoric acid commonly contains trace elements of Polonium-210(Po-210). Po-210 is a radioactive element and a source of alpharadiation emissions. Levels of Po-210 and, in proportion, alphaemissions, vary depending on the content of Po-210 in phosphorous usedto make the acid solution. The Po-210 present in any deposit ofphosphorous is a function of the natural decay of U-238. Po-210 Levelsmay vary greatly from an imperceptible emission level to significantalpha emissions.

It is well known in the industry that alpha particle emissions are oneof several known causes of soft errors in LSI memory devices. Softerrors have been defined to be random, non-recurring single bit errorsin memory devices. They are not permanent, i.e., no physical defects areassociated with the failed bit. A bit showing a soft error is completelyrecovered by the following write cycle, for example in a dynamic memorydevice where refresh of memory stored data occurs every severalnanoseconds.

Other identified causes of soft errors are system noise, voltagemarginality, sense amplifiers and pattern sensitivity, all statisticalpredictors of the rate at which soft errors will occur (SER).

Po-210, when present in phosphoric acid in the etching process, has beenidentified as a source of alpha particle emissions. Po-210 has anaffinity for Silicon (Si) in the acid bath and plates onto the surfaceof the Si wafers which are being etched for production of finished LSImemory and other devices. The Po-210 remains bonded at surface sites onthe silicon wafer, through later subsequent manufacturing steps. Thefinished LSI circuit or die results in a memory device with internallyemitted alpha particles. The alpha particles are emitted by the Po-210contaminating the die Thus, the memory device becomes itself a source ofcontribution of SER.

SUMMARY OF THE INVENTION

Silicon is employed as a "getter" to attract Polonium 210 (Po-210)molecules and remove them from the liquid phosphoric acid bath. Getteris used in this instance to describe silicon as the attracting agentwhich reacts with the noble metal, Po-210 in this example. The mechanismfor this attraction is described as follows: The getter is the vehiclefor removing the contaminating noble metal. Gettering refers to the actof attracting and removing the trace elements through the reduction ofthe trace elements onto silicon. Noble metals include silver, gold,copper, platinum, palladium, mercury, selenium and bismuth, in additionto Polonium.

In accordance with the teachings of the present invention, a method ofremoving noble metal trace elements from a mineral acid is disclosed,comprising the steps of heating a liquid mineral acid in a container,inserting a silicon getter into the acid in fluid contact with the acid,and removing the silicon getter from said mineral acid.

A method is set forth, wherein the liquid acid is heated to atemperature of approximately 145-150 degrees C. The mineral acid isselected from the group consisting of phosphoric acid or sulfuric acid,and any acidic solution of pH 6 or lower. The mineral acid contains atleast a trace of a noble metal. The noble metal is selected from thegroup consisting of Polonium, gold, silver, platinum, copper, palladium,mercury, selenium and bismuth. The getter is inserted in the liquidmineral acid for at least thirty minutes.

A method of adsorbing Po-210 with a silicon getter from a phosphoricacid solution comprises the steps of providing liquid phosphoric acid ina container, heating the container of liquid phosphoric acid to between145 degrees C. and 150 degrees C., inserting at least one siliconarticle into an open vessel, placing the vessel into the acid bath atleast until reaching process temperature, soaking the silicon article inthe liquid phosphoric acid and removing the vessel from the container.

An improved method of manufacturing electronic semiconductor integratedcircuits is also disclosed. The method of substantially eliminatingα-particle emissions attributable to manufacturing materials, comprisesthe steps of providing a quantity of phosphoric acid having at least aportion composed of Polonium-210, then heating the quantity ofphosphoric acid to a temperature suitable for removal of silicon nitridefrom a silicon wafer, inserting a silicon getter into the phosphoricacid during or following the heating step, next removing the silicongetter from the phosphoric acid, and etching silicon nitride from asemiconductor wafer having wiring paths defined thereon.

It is therefore an object of the present invention to provide a methodof eliminating alpha particle emissions from phosphoric acid whilesimultaneously removing Po-210 from the acid solution.

It is further an object of the present invention to provide a method ofdecontaminating phosphoric acid for use in the manufacture of siliconLSI memory devices.

Another object of the present invention is to provide a method ofplating noble metals onto silicon from an acidic solution.

Yet another object of the present invention is to provide a method ofmanufacturing a LSI memory device substantially free of soft errors dueto internal alpha particle radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a top view of the process tank placed within areservoir tank adjacent to pump and filter.

FIG. 2 illustrates the top view of FIG. 1 with a cover placed over topof the pump and filter sections.

FIG. 3 is a cross-sectional profile view taken along the lines 3--3 inFIG. 1.

FIG. 4 is an illustration of a vessel containing silicon wafers.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following experiments were devised to illustrate and to prove theeffectiveness of the present invention.

EXPERIMENT NO. 1

In this experiment, a set of four circular silicon wafers measuring 6"in diameter and 675 microns in thickness were dipped in a bath of freshphosphoric acid at 145 degrees to 150 degrees C. for thirty minutes. Thefirst four wafers were then removed. The term "wafer" as used in thisdescription of this invention refers to a wafer of the type used in themanufacturer of packaged semiconductor device. The dimensions are 6"diameter circular silicon wafer of 650 to 700 micron thickness.!

The phosphoric acid bath was then gettered by dipping a batch of fiftysilicon wafers for thirty minutes. The temperature was held constant at145°-150° C. The fifty wafers used to buffer the acid bath are definedas getters. The getters were then removed.

A second group of four wafers was then dipped in the same bath for 30minutes at the same temperature.

α-particle emissions were next measured by placing the first group offour wafers in a cloud chamber for nine hours. A reading of thecumulative α-particles counts was taken once every hour for nineconsecutive hours. Measurements were made using a Spectrum Science Co.Model 850 radiation analyzer. The cloud chamber was purged. Then thesecond group of four wafers was placed in the cloud chamber. α-particleemissions were measured using the same procedure. The results are setforth in Table 1.

                  TABLE 1                                                         ______________________________________                                                                    α-particle                                  After        α-particle Emissions                                                                   Group 2 after                                     Hour         Group 1 before buffering                                                                     buffering                                         ______________________________________                                        1            382            209                                               2            707            400                                               3            1053           585                                               4            1371           774                                               5            1681           971                                               6            2007           1161                                              7            2312           1353                                              8            2617           1518                                              9            2919           1706                                              Average Counts                                                                             77.4           46.6                                              per wafer-hour                                                                ______________________________________                                    

EXPERIMENT NO. 2

Eight gallons of phosphoric acid was mixed in solution. Half was placedin one tank and the remainder in a second tank. The mixture was dividethis way in order to ensure that the chemical reagents in each tank wereidentical. Both tanks were then gradually heated to 145° to 150° C.,over approximately a thirty minute period.

The first tank was not treated with getter wafers before introducing thesample production wafers. Two sample production wafers were placed inthe heated phosphoric acid bath for thirty minutes and then removed. α-emission measurements were made after placing the wafers in a cloudchamber. Using a Spectrum Science Model 850 radiation detector,measurements were taken at one hour intervals for nine hours. Theresults are listed in Table 1A.

The second tank was first gettered with fifty getter wafers. Afterheating to the desired process temperature above, the getter wafers wereplaced in a vessel and submerged in the phosphoric acid bath of tank #2.After thirty minutes elapsed, the vessel was removed. Then two sampleproduction wafers were placed into the buffered phosphoric acid bath.Alpha emissions were then measured in the same manner as described forthe first tank sample wafers. The results are listed in Table 1B.

The average counts per wafer-hour was calculated as a measure ofradioactivity level for comparison purposes. The calculation is made bysubtracting the 4th hour cumulative emissions from the 9th hourcumulative total, in order to eliminate any possible "background" orambient radioactive contribution due to other sources. Radon gas is oneexample of ambient background radiation contribution. The difference inthose two readings yields a 5-hour cumulative total α-emission which issolely attributable to contaminants in the processmaterial--i.e.--Po-210 present in the phosphoric acid. The total is thendivided by 5 to yield an average hourly emission, then divided again bytwo to yield a per-wafer, per hour measurement.

    ______________________________________                                        TABLE 1A                                                                      ______________________________________                                                         Alpha Particle                                               After            Emissions Measured                                           Hour No.         (Acid Bath Not Buffered)                                     ______________________________________                                        1                149 counts                                                   2                234                                                          3                346                                                          4                460                                                          5                565                                                          6                665                                                          7                772                                                          8                861                                                          9                950                                                          Average          49 counts/wafer-hour                                         ______________________________________                                        TABLE 1B                                                                      ______________________________________                                                         Alpha Particle                                               After            Emissions Measure                                            Hour No.         (Acid Bath Buffered)                                         ______________________________________                                        1                 68 counts                                                   2                153                                                          3                224                                                          4                304                                                          5                368                                                          6                427                                                          7                501                                                          8                563                                                          9                635                                                          Average          33.1 counts/wafer hours                                      ______________________________________                                    

This experiment reveals that emissions from sample production wafers intank two after buffering measured approximately 65% of the emissionlevels of the identical wafers from tank one.

Each tank contained identical solutions of phosphoric acid, and thesample production wafers tested in each tank were selected from the sameproduction batch.

A second part of the same experiment consisted of measuring the alpharadiation levels of the getter wafers--the group of 50 wafers used tobuffer tank number two. Two wafers (No. 9 and 10) were randomly selectedfrom this group and measured for alpha radiation using the samemeasurement procedure as used in the first part of this experiment.Table 2A sets forth the results of this phase of the experiment.

                  TABLE 2A                                                        ______________________________________                                                After                                                                              Alpha                                                                    Hour Emission                                                                 No.  Counts                                                           ______________________________________                                                1    105                                                                      2    186                                                                      3    269                                                                      4    362                                                                      5    438                                                                      6    531                                                                      7    602                                                                      8    697                                                                      9    784                                                              ______________________________________                                    

Consistent with the results in tables 1A and 1B, this establishes thatthe source of alpha emissions is gettered by the Si wafers placed in thevessel. While the Po-210 is adsorbed by fifty of the gettering wafers ina half hour soak period, it is expected that the larger number pervolume of acid of getter wafers will adsorb Po-210 a rate (42.2) equalto or less than the two wafers placed in the unbuffered solution of tankNo. 1(49).

EXPERIMENT NUMBER 3

In a third experiment, the effect of gettering as a function of time wasdetermined. Results showed that the longer the silicon gettering waferswere immersed in the heated acid bath, the lower the Po-210contamination in the acid solution after the gettering wafers areremoved.

A tank of liquid phosphoric acid was heated to 145°-150° C. From a batchof fifty wafers, two were selected at random (Nos. 13 and 14). These twowafers were submerged for thirty minutes and removed, then cleaned,dried and stored in one airtight flat-pack container for a short time.Using the same measurement technique, α-particles were measured at 45.5counts per wafer-hour.

The same acid bath at process temperature was then gettered by insertingthe vessel into the bath for one hour. The vessel contained fiftygettering wafers. After one hour, the vessel was pulled from the tankand two production wafers were placed in the tank for one half hour.α-emissions for these two wafers were measured at 32.4 counts perwafer-hour.

The same acid bath was again gettered with a second batch of fiftysilicon wafers for a second one hour interval. The vessel containing thegetters was removed. Two production wafers were inserted in the bath forone half hour and removed. α-particle emissions were recorded as 26.6counts per wafer hour.

By buffering the acid bath with fifty getter wafers for one hour a 28.8%reduction in α-particles was realized; after then buffering the samebath for a second hour, an overall reduction in α-particle emission of41.5% was measured.

EXPERIMENT 4

In order to determine the relation of surface area of the silicon gettermaterial to the efficiency of α-particle (Po-210) removal, a fourthexperiment was conducted. In this experiment, a quantity of high-qualitysilicon beads weighing 500 grams were used instead of the 50 wafers(weight=1500 grams) as in the previous experiments. The silicon beadsare spherical, so it was assumed--without measuring--that these wouldpermit significantly greater fluid contact with the phosphoric acid.

A batch of phosphoric acid was mixed and divided into two separatetanks, then both tanks were heated to 145°-150° C. One tank was bufferedwith silicon getter beads for approximately four to five hours atprocess temperature. The second tank was not buffered at all, andremained at process temperature for the same time interval.

Five production wafers were then placed in each tank and removed afterthirty minutes. The wafers were then placed in the cloud chamber.α-particles were measured again with the Spectrum Model 850 radiationanalyzer. For this experiment, measurements were recorded once everyhour for fifteen consecutive hours. Contribution due to backgroundemissions were compensated by subtracting hour 4 reading from hour 15reading. The counts per wafer hour were determined by dividing thedifference by eleven (hours); and dividing again by five (wafers)according to the same method as experiment number 1! to arrive at thecomparison figures. Table 4 shows the readings for each set of wafersaccording to each of fifteen hours.

                  TABLE 4                                                         ______________________________________                                        Hour           Unbuffered                                                                              Buffered                                             ______________________________________                                         1             1008      168                                                   2             2004      290                                                   3             2953      408                                                   4             3879      514                                                   5             4845      607                                                   6             5793      722                                                   7             6755      607                                                   8             7682      722                                                   9             8630      828                                                  10             9553      1171                                                 11             10,507    1282                                                 12             11,459    1412                                                 13             12,399    1542                                                 14             13,352    1658                                                 15             14,286    1768                                                 Counts per     189.2     22.8                                                 wafer-hour                                                                    ______________________________________                                    

Results indicated reduction of α-particle emissions of over 80% in thesilicon wafers after buffering the phosphoric acid bath compared toα-particle emissions in the silicon wafers when the acid bath was notbuffered.

It should be noted that each batch of phosphoric acid may have high orlow Polonium-210 content, depending on the source where the phosphorouswas mixed. Thus, the magnitude of the various readings, for buffered andunbuffered results is less significant than their percentagedifferences.

Referring to FIGS. 1,2 and 3, a preferred embodiment of the presentinvention is shown. Recirculating bath filter system 10 includesreservoir tank 12. Heater 14 is located adjacent to the bottom of tank12. Heater 14 may be of insertion type or immersion type. Process tank16 fits into reservoir 12. Reservoir 12 and process tank 16 arepreferably made of teflon to withstand the corrosive liquids containedwithin them.

Reservoir tank 12 is filled with liquid phosphoric acid 20, then heatedto 145°-150° C. The process temperature ideally suited to strippingnitride from the surface of silicon when etching microscopic circuitsonto silicon wafer surfaces. During the heating cycle and after,phosphoric acid 20 is circulated through silicon media 28 in filterhousing 24. Pump 22 circulates acid 20 through filter housing 24 andback to reservoir 12 through conduits 26, 26A (not shown) made of teflonfittings for supply and return of acid 20.

Silicon media 28 getters po-210 from the phosphoric acid. After apredetermined interval has elapsed--usually thirty to forty-fiveminutes--process temperature in the bath is achieved and Pl-210 isgettered from the acid. Vessel 32 is substantially immersed in theliquid acid bath 20 by placing it into process tank 16. Vessel 32contains silicon wafers 34 which are then cleaned and etched formanufacture of integrated circuit devices. Note the distinction betweengettering wafers 36 and silicon wafers used in production of finaldevices.

Referring next to FIG. 4, vessel 32 is illustrated for use in analternative method of gettering Po-210 from phosphoric acid. In thismethod, gettering wafers 36 are placed in vessel 32 then submerged inreservoir tank 12 while the bath is heated to process (145°-150° C.)temperature from ambient. This method is useful where it may beimpractical or uneconomical to equip tank 12 with filter housing 24 andpump 22, and associated plumbing. Gettering wafers 36 replace siliconmedia 28 from filter housing 24, and are manually inserted and removedfrom the acid reservoir 20 to getter Po-210 before the productionsilicon wafers are dipped in reservoir 20. Vessels 32 are shaped like adish rack so that the flat gettering wafers 36 may be supportedvertically in a row, typically twenty five to a vessel.

As previously described, noble metals in an acidic solution will beadsorbed by--or plate onto--a reducing agent such as silicon. Forsubstantial plating to occur, the pH of the solution must be 6 or lower.Optimal plating conditions exist in a mineral acid bath such asphosphoric or sulfuric. While heating the bath increases the rate atwhich noble metals are adsorbed by the reducing agent, it is notnecessary to heat the acid solution in order for adsorption to occur.

Similarly, the time of exposure of the silicon reducing agent is setforth in the experiments above, as a means of illustrating effects andfor measuring the effects It should be noted that reduction of the noblemetal begins immediately at room or ambient temperature, so long as theacid is liquid to allow for immersion of the reducing agent.

Depending on the quantity of acid solution one desires to decontaminateof noble metal traces, temperature and time of exposure may be increasedor decreased to yield timely and efficient results. Experiments onethrough four above were conducted with quantities of eight gallons ofphosphoric acid. The same effect will be realized on much greaterscales. For example, in a large storage facility for storing acid, itmay be impractical or uneconomical to heat large storage tanks. It isnot necessary however to add heat where the silicon may be exposed forsignificantly longer intervals.

According to the provisions of the Patent Statutes, we have explainedthe principal, preferred construction and mode of operation of ourinvention and have illustrated and described what we now consider torepresent its best embodiments. However, it should be understood that,within the scope of the appended claims, the invention may be practicedotherwise than as specifically illustrated and described.

We claim:
 1. A method for removing a radioactive contaminant from afluid, comprising the following steps:a) providing contact betweensilicon and the fluid; b) allowing sufficient time for the radioactivecontaminant to adhere to the silicon; c) maintaining a temperature ofthe fluid within a range of temperatures between 145 and 150 degreescentigrade during said time defined by said step of allowing; and d)separating the fluid and the silicon having the adhered radioactivecontaminant.
 2. The method as specified in claim 1, wherein the fluid isan acidic solution.
 3. The method as specified in claim 1, furthercomprising inserting the silicon into a vessel containing the fluidduring said step of providing.
 4. The method as specified in claim 3,further comprising removing the silicon having the adhered radioactivecontaminant from the vessel during said step of separating.
 5. A methodfor removing a radioactive contaminant from an acidic solution,comprising the following steps:a) inserting silicon into a container influid contact with the acidic solution; b) heating the acidic solutionto a temperature at which the silicon attracts the radioactivecontaminant, said temperature falling within a range of temperaturesbetween 145 and 150 degrees centigrade; and c) removing the silicon andthe radioactive contaminant from the container.
 6. A method forfabricating a semiconductor integrated circuit, comprising the followingsteps:a) providing a phosphoric acid solution containing polonium-210;b) heating said phosphoric acid solution to a temperature suitable forattracting the polonium-210 to silicon; and c) inserting silicon intosaid phosphoric acid solution; and d) heating said silicon to saidtemperature such that the polonium-210 is attracted to said silicon. 7.The method as specified in claim 6, further comprising removing saidsilicon from the phosphoric acid solution thereby removing at least aportion of the polonium-210 from the phosphoric acid solution.
 8. Themethod as specified in claim 7, further comprising etching a materialused to fabricate the semiconductor integrated circuit with thephosphoric acid solution subsequent to said step of removing.